My research focuses on understanding how changing environmental (e.g. terrestrial nutrient loading) and biotic (e.g. trophic structure) factors influence the transformation and transport of elements within freshwater ecosystems. I use a variety of research approaches, including stable isotopes to measure ecosystem-scale fluxes and next generation sequencing techniques to assess microbial community responses. I use experimental manipulations as well as natural studies in my research, at scales ranging from laboratory microcosms to whole ecosystems. Here is a summary of current and past projects.
Tree hole in beech tree. Photo credit: Jen Kirk
Insect-microbe interactions in water-filled tree holes
Post-doctoral research at Michigan State University Collaborators: Ned Walker, Mike Kaufman
Approximately 40% of mosquito species complete the larval stage of their life cycle in water-filled containers including those that carry many human diseases. Water-filled tree holes are ideal habitats for these mosquitoes. The food webs of these tree holes are fueled by the decomposition of dead plant material by microbes, which are consumed by mosquito larvae.
How do leaf litter quantity and quality interact to influence mosquito production? The amount of leaf litter per larvae is an important driver of mosquito production. This relationship suggests density dependent population dynamics, but the quality of detritus also affects production. Leaves in aquatic ecosystems undergo a predictable sequence of quality changes, or conditioning, driven by leaching of soluble materials, microbial colonization, and carbon use. My research investigated the effect of leaf conditioning on the density dependent resource limitation commonly observed in tree holes.
We found that the degree of leaf conditioning modified the effects of larval density and resource availability on cohorts of Ae. triseriatus (the Eastern tree hole mosquito) in laboratory mesocosms. Density dependent resource limitation was important when larvae grew on leaves in the early stages of conditioning but not when they were grown on well-conditioned leaves.
These results have implications for how we study populations of medically-relevant mosquito species. Laboratory mesocosms are commonly used to investigate these species. However, these experiments often use detritus conditioned for 10 d or less. Our results suggest that the use of poorly conditioned detritus may lead to an overestimation of the importance of competition as a driver of mosquito production.
How does grazing by mosquito larvae influence microbial communities? Consumer effects on freshwater microbial communities have largely been investigated in herbivore-producer communities. Studying the response of heterotrophic microbial communities to grazing has been difficult due to the difficulties in assessing the diversity and composition of these communities. Advancements in next generation sequencing platforms and downstream analysis programs are overcoming some of these challenges.
We used tree hole habitats to investigate how grazing by mosquito larvae alters the successional trajectory of leaf-associated bacterial and fungal communities. We exposed early and late stage microbial communities to varying intensities of larval feeding (0, 15, 20, 30, 40 larvae/g leaf). We sequenced the V4 region of the bacterial/archeal 16s gene and the V9 regions of the fungal 18s gene and ITS1 region to describe microbial community composition.
Bacterial and fungal communities followed different successional trajectories. Well-established bacterial communities were dramatically more diverse than early-stage communities while fungal diversity was similar between the two community stages. Predation by mosquito larvae did not affect successional trajectories of either group. These findings suggest that bacteria and fungi on the same substrate respond to different successional drivers and that predation has little influence on these trajectories.
How do benthic processes influence mosquito production from tree holes? Water-sediment interfaces are hotspots for biogeochemical processes in freshwaters and nutrient exchange across these boundaries may support secondary production. Many medically-relevant mosquito species rely on water-filled containers as larval habitat. These containers, particularly natural tree holes, often include an anoxic sediment layer. Studies of mosquito production from these habitats have largely ignored sediment processes as larvae forage in the water column.
We compared the emergence, biomass, and development time of the eastern tree hole mosquito (Aedes triseriatus) from laboratory containers with and without sediment. The presence of sediment consistently affected mosquito production. Containers with sediment produced more adults that were heavier than those produced from containers without sediment. However, mosquito development time was longer in containers with sediment. We propose several potential mechanisms by which microbial activity within sediments may affect mosquito production, including the removal of toxins from the water column and the export of limiting nutrients or microbial biomass to the water column. We suggest that coupled aerobic and anaerobic nitrogen transformations are particularly important in tree holes. Our results demonstrate that processes occurring within the sediment do impact mosquito production, despite the pelagic nature of mosquito larvae.
Relevant Publications:
Norman, B. C. and E. D. Walker. Effect of predation on leaf-associated microbial communities in different stages of development. In review, Aquatic Microbial Ecology
Norman, B. C. and E. D. Walker. Conditioning of leaf detritus modulates density-dependent growth of Aedes triseriatus larvae (Diptera: Culicidae). Journal of Medical Entomology 55:342-350. DOI: 10.1093/jme/tjx209
Hugh White Creek, Coweeta LTER: a typical Appalachian headwater stream with low N availability. Photo credit: Jack Webster
N Cycling During Decomposition in Streams
Doctoral research at Virginia Tech Collaborators: Jack Webster, Fred Benfield, Maury Valett, Jeb Barrett, Erika Bilger
Where do leaf-associated microbes get their N? Allochthonous carbon inputs are important sources of energy for forested headwater streams. Microbes that colonize dead leaves in streams obtain nutrients either from the water column or from the leaves. I hypothesized that the relative importance of these sources for microbial growth would change temporally as decomposition progressed and the availability of leaf N changed, and spatially among streams with different exogenous N availability.
I placed packs of leaves enriched with 15-N in 5 Appalachian headwater streams to determine which source microbes used to support growth. We found that microbes rely heavily on leaf-derived N during the initial stages of decomposition but switch to immobilizing dissolved N from the water column fairly quickly. We also found that dissolved N availability had little influence on the % microbial N obtained from the water. Microbes relied on water column N even in streams with low N availability, perhaps because water column N, while scarce, is consistently available due to replenishment from upstream.
Do macroinvertebrates affect N cycling and leaf breakdown rates in streams? Animals in freshwater habitats are among the most threatened by global changes including land use shifts, eutrophication, and over fishing. My research asked how animals interact with microbes to influence ecosystem function. Animals may influence microbial activity through consumption of microbial biomass and by altering nutrient availability via consumer nutrient recycling (CNR).
We investigated the effects of amphipods and tipulids on N uptake during leaf decomposition by leaf-associated microbes under conditions of ambient and enriched N availability in artificial streams. Nitrogen enrichment generally increased red maple breakdown rates, while shredder influence on leaf breakdown was taxon specific. Feeding by tipulids increased red maple breakdown rate compared with amphipods, although not significantly. While feeding by amphipods did not directly influence breakdown rates, amphipod nutrient recycling stimulated leaf breakdown rate by the same magnitude as the N-enrichment treatment.
Relevant Publications:
Cheever, B. M. and J. R. Webster. 2014. Effects of consumers and nitrogen availability on heterotrophic microbial activity during leaf decomposition in headwater streams. Freshwater Biology 59:1768-1780
Cheever, B. M., J. R. Webster, E. E. Bilger, and S. A. Thomas. 2013. The relative importance of exogenous and substrate derived nitrogen for microbial growth during leaf decomposition. Ecology 94: 1614-1625.
Cheever, B. M., E. B. Kratzer, and J. R. Webster. 2012. Immobilization and mineralization of N and P by heterotrophic biofilms during leaf decomposition. Freshwater Science 31: 133-147.
Silver nanoparticle solution added to a lake mesocosm
Lake Ecosystem Response to Silver Nanoparticles
Post-Doctoral research at Trent University Collaborators: Jenn Vincent, Graham Blakelock, Lindsay Furtado, Paul Frost, Maggie Xenopoulos
Silver nanoparticles (AgNPs) are found in many consumer products including clothing, children's toys, and medical supplies. These particles are used because they have antimicrobial properties. Therefore, the release of AgNPs into freshwater environments is an emerging environmental concern as they have the potential to disrupt microbe-driven ecosystem functions.
How does P availability modify impacts of AgNPs on periphyton? AgNP toxicity in freshwater may be influenced by ambient physico-chemical conditions. Eutrophication is occurring in many lakes. We used nutrient diffusing substrates to investigate the main and interactive effects of AgNP exposure and P availability on periphyton biomass and stoichiometry in lakes spanning a range of dissolved carbon concentration. We found that P enrichment increased the toxicity of AgNPs, particularly in the clear water lake. These results contrast with the interaction of nutrients and other metals, highlighting the importance of in situ investigations of emerging contaminants.
How does AgNP concentration, capping agent, and dosing regimen affect natural lake ecosystems? The effects of AgNPs have largely been investigated using short-term studies with model organisms in simplified media. We used in situ mesocosms to investigate AgNP effects on a natural lake ecosystem. We added AgNPs in varying concentrations with different capping agents in a pulse and chronic exposure regimen and monitored responses of bacteria, algae and zooplankton communities as well as functions such as community respiration, gross primary production, net ecosystem production, and organic matter decomposition. Our preliminary results suggest that basal trophic level standing stocks and ecosystem functions were maintained or recovered quickly from AgNP exposure while zooplankton communities were more strongly affected.
Relevant Publications:
Norman, B. C., M. A. Xenopoulos, E. Hoque, L. Furtado, P. C. Frost, C. Metcalfe, and H. Hintelmann. Community responses and silver accumulation in a planktonic food web exposed to silver nanoparticles. In revision, FACETS
Vincent, J. L., M. J. Patterson, B. C. Norman, E. P. Gray, J. F. Ranville, A. B. Scott, P. C. Frost, and M. A. Xenopoulos. 2017. Chronic and pulse exposure effects of silver nanoparticles on natural lake phytoplankton and zooplankton. Ecotoxicology 26:502–515
Blakelock, G. C., M. A. Xenopoulos, B. C. Norman, J. L. Vincent, and P. C. Frost. 2016. Effects of silver nanoparticles on bacterioplankton in a boreal lake. Freshwater Biology 61:2211-2220
Furtado, L. M., B. C. Norman, M. A. Xenopoulos, P. C. Frost, C. D. Metcalfe, and H. Hintelmann. 2015. Environmental fate of silver nanoparticles in boreal lake ecosystems. Environmental Science & Technology 49: 8441-8450
Norman, B. C., M. A. Xenopoulos, D. Braun, and P. C. Frost. 2015. Phosphorus availability alters the effects of silver nanoparticles on periphyton growth and stoichiometry. PLoS ONE 10(6): e0129328. doi:10.1371/journal.pone.0129328
Furtado, L. M., M. E. Hoque, D. Mitrano, J. Ranville, B. M. Cheever, P. C. Frost, M. A. Xenopoulos, H. Hintelmann, and C. D. Metcalfe. 2014. The persistence and transformation of silver nanoparticles in littoral lake mesocosms monitored using various analytical techniques. Environmental Chemistry 11:419-430.